A key union representing Alitalia workers has backed a rescue plan by the Italian investor group CAI.
Alitalia, the airline which is struggling to survive, has been given a temporary reprieve by the Italian civil aviation authority.
The Cgil union signed an agreement with the struggling airline on Thursday at a meeting in Rome.
ENAC said it would not revoke Alitalia's licence to fly "for now" after Italy's biggest union agreed to a revised rescue plan.
Any deal must still win the support of unions representing pilots and flight assistants if Alitalia is to survive.
The authority had threatened to ground Alitalia's planes unless it presented an agreed plan by the end of Thursday.
Italy's civil aviation authority had threatened to ground Alitalia planes if the airline failed to present a rescue plan by the end of the day.
Unions representing pilots and crew are yet to agree to the deal, however.
Cgil said that it had "signed an agreement with Alitalia", after winning concessions on pay, leave, contracts and temporary jobs.
"For now, the feared risk of yanking the licence isn't there," said ENAC civil aviation chief Vito Riggio.
Four of the nine unions have now agreed to CAI's terms.
But he added that ENAC "will monitor Alitalia's financial situation month by month".
The remaining five unions, acting for pilots and flight attendants, are meeting the investor group and the airline's administrators later on Thursday.
The CGIL union said it had signed an agreement with Alitalia, after winning concessions on pay, leave, contracts and temporary jobs.
After weeks of wrangling, Italian Premier Silvio Berlusconi is attempting to get the remaining unions to agree to CAI's offer.
Four of the nine unions have now agreed to the terms of the rescue deal proposed by investors group CAI. CGIL has urged the remaining five to end their opposition.
Losing money
Alitalia's government-appointed administrator Augusto Fantozzi said ticket sales had fallen by 100,000 in September, hit by the uncertainty surrounding the airline.
The development is good news for the debt-laden carrier, which is estimated to be losing more than 2m euros ($2.9m; £1.58m) a day.
CAI withdrew its offer for Alitalia last week after some unions rejected the deal because of fears about jobs cuts.
The group aims to inject $1.4bn (£755m) into the bankrupt airline, sell off unprofitable assets and merge the rest with Italy's second biggest carrier Air One.
Earlier this week, the administrator for the airline placed advertisements in four newspapers in a bid to attract buyers for the carrier.
